Magnetoresistive structure comprising ferromagnetic thin films and intermediate alloy layer having magnetic concentrator and shielding permeable masses

ABSTRACT

A magnetoresistive layered structure having on a substrate two or more magnetoresistive, anisotropic ferromagnetic thin-films each two of which are separated by an intermediate layer on a substrate of less than 50 Å thickness formed of a substantially nonmagnetic, conductive alloy having two immiscible components therein. A further component to provide temperature stability in some circumstances is to be at least partially miscible in the first two components. Such structures can be formed as a sensor by having them electrically connected together with one positioned in a gap between magnetic material masses and one shielded by one of such masses. The magnetic material mass being used for shielding can be divided into two masses with one of those masses farthest from the gap serving as the shield.

This invention was made in part with Government support under ContractDASG 60-92-C-0073 and under Contract DASG 60-93-C-0042 both awarded bythe U.S. Army. The Government has certain rights in the invention.

This application is a continuation-in-part application ofcontinuation-in-part application Ser. No. 08/182,614, filed Jan. 18,1994 which is a continuation-in-part of application Ser. No. 07/976,905,filed Nov. 16, 1992, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to ferromagnetic thin-film structures and,more particularly, to ferromagnetic thin-film structures exhibitingrelatively large magnetoresistive characteristics.

Many kinds of electronic systems make use of magnetic devices. Digitalmemories are used extensively in digital systems of many kinds includingcomputers and computer systems components, and digital signal processingsystems. Such memories can be advantageously based on the storage ofdigital bits as alternative states of magnetization in magneticmaterials in each memory cell, particularly in cells using thin-filmmagnetic materials, resulting in memories which use less electricalpower and do not lose information upon removals of such electricalpower.

Magnetometers and other magnetic sensing devices are also usedextensively in many kinds of systems including magnetic disk memoriesand magnetic tape storage systems of various kinds. Such devices provideoutput signals representing the magnetic fields sensed thereby in avariety of situations.

Such memory cells and sensors can often be advantageously fabricatedusing ferromagnetic thin-film materials, and are often based onmagnetoresistive sensing of magnetic states, or magnetic conditions,therein. Such devices may be provided on a surface of a monolithicintegrated circuit to provide convenient electrical interconnectionsbetween the device and the operating circuitry therefor.

Ferromagnetic thin-film memory cells, for instance, can be made verysmall and packed very closely together to achieve a significant densityof information storage, particularly when so provided on the surface ofa monolithic integrated circuit. In this situation, the magneticenvironment can become quite complex with fields in any one memory cellaffecting the film portions in neighboring memory cells. Also, smallferromagnetic film portions in a memory cell can lead to substantialdemagnetization fields which can cause instabilities in themagnetization state desired in such a cell.

These magnetic effects between neighbors in an array of closely packedferromagnetic thin-film memory cells can be ameliorated to aconsiderable extent by providing a memory cell based on an intermediateseparating material having two major surfaces on each of which ananisotropic ferromagnetic memory thin-film is provided. Such anarrangement provides significant "flux closure," i.e. a more closelyconfined magnetic flux path, to thereby confine the magnetic fieldarising in the cell to affecting primarily just that cell. This isconsiderably enhanced by choosing the separating material in theferromagnetic thin-film memory cells to each be sufficiently thin.Similar "sandwich" structures are also used in magnetic sensingstructures.

In the recent past, reducing the thicknesses of the ferromagneticthin-films and the intermediate layers in extended "sandwich" structureshaving additional alternating ones of such films and layers, i.e.superlattices, have been shown to lead to a "giant magnetoresistiveeffect" being present. This effect yields a magnetoresistive responsewhich can be in the range of up to an order of magnitude greater thanthat due to the well-known anisotropic magnetoresistive response.

In the ordinary anisotropic magnetoresistive response, varyingdifferences between the direction of the magnetization vector in theferromagnetic film and the direction of the sensing current passedthrough the film lead to varying differences in the effective electricalresistance in the direction of the current. The maximum electricalresistance occurs when the magnetization vector in the film and thecurrent direction are parallel to one another, while the minimumresistance occurs when they are perpendicular to one another. The totalelectrical resistance in such a magnetoresistive ferromagnetic film canbe shown to be given by a constant value, representing the minimumresistance, plus an additional value depending on the angle between thecurrent direction in the film and the magnetization vector therein. Thisadditional resistance follows a square of the cosine of that angle.

As a result, operating external magnetic fields can be used to vary theangle of the magnetization vector in such a film portion with respect tothe easy axis of that film portion which comes about because of ananisotropy therein typically resulting from depositing the film in thepresence of a fabrication external magnetic field oriented in the planeof the film along the direction desired for the easy axis in theresulting film. During subsequent operation of the device with theresulting film, such operating external magnetic fields can vary theangle to such an extent as to cause switching of the film magnetizationvector between two stable states which occur as magnetizations orientedin opposite directions along that easy axis. The state of themagnetization vector in such a film portion can be measured, or sensed,by the change in resistance encountered by current directed through thisfilm portion. This arrangement has provided the basis for aferromagnetic, magnetoresistive anisotropic thin-film to serve as partof a memory cell.

In contrast to this arrangement, the resistance in the plane of aferromagnetic thin-film is isotropic with respect to the giantmagnetoresistive effect rather than depending on the direction of asensing current therethrough as for the anisotropic magnetoresistiveeffect. The giant magnetoresistive effect has a magnetization dependentcomponent of resistance that varies as the cosine of the angle betweenmagnetizations in the two ferromagnetic thin-films on either side of anintermediate layer. In the giant magnetoresistive effect, the electricalresistance through the "sandwich" or superlattice is lower if themagnetizations in the two separated ferromagnetic thin-films areparallel than it is if these magnetizations are antiparallel, i.e.directed in opposing directions. Further, the anisotropicmagnetoresistive effect in very thin-films is considerably reduced fromthe bulk values therefor in thicker films due to surface scattering,whereas very thin-films are a fundamental requirement to obtain asignificant giant magnetoresistive effect.

In addition, the giant magnetoresistive effect can be increased byadding further alternate intermediate and ferromagnetic thin-film layersto extend a "sandwich" or superlattice structure. The giantmagnetoresistive effect is sometimes called the "spin valve effect" inview of the explanation that a larger fraction of conduction electronsare allowed to move more freely from one ferromagnetic thin-film layerto another if the magnetizations in these layers are parallel than ifthey are antiparallel with the result that the magnetization states ofthe layers act as sort of a valve.

These results come about because of magnetic exchange coupling betweenthe ferromagnetic thin-films separated by the intermediate layers, theseintermediate layers typically formed from a nonferromagnetic transitionmetal. The effect of the exchange coupling between the ferromagneticthin-film layers is determined to a substantial degree by the thicknessof such an intermediate layer therebetween. The effect of the couplingbetween the separated ferromagnetic thin-film layers has been found tooscillate as a function of this separation thickness between theselayers in being ferromagnetic coupling (such that the magnetizations ofthe separated layers are parallel to one another) and antiferromagneticcoupling (such that the magnetizations of the separated layers areopposed to one another, or antiparallel to one another). Thus, for someseparation thicknesses, the layer coupling will be of zero value betweenextremes of such oscillations.

Exhibiting the giant magnetoresistive effect in a superlatticestructure, or in an abbreviated superlattice structure formed by a threelayer "sandwich" structure, requires that there be arrangements inconnection therewith that permit the establisltment alternatively ofboth parallel and antiparallel orientations of the magnetizations in thealternate ferromagnetic thin-film layers therein. One such arrangementis to have the separated ferromagnetic thin-films in the multilayerstructure be antiferromagnetically coupled but to a sufficiently smalldegree so that the coupling field can be overcome by an externalmagnetic field.

Another arrangement is to form the ferromagnetic thin-film layers withalternating high and low coercivity materials so that the magnetizationof the low coercivity material layers can be reversed without reversingthe magnetizations of the others. A further alternative arrangement isto provide "soft" ferromagnetic thin-films and exchange couple everyother one of them with an adjacent magnetically hard layer (forming aferromagnetic thin-film double layer) so that the ferromagnetic doublelayer will be relatively unaffected by externally applied magneticfields even though the magnetizations of the other ferromagneticthin-film layers will be subject to being controlled by such an externalfield.

One further alternative arrangement, related to the first, is to providesuch a multilayer structure that is, however, etched into strips suchthat demagnetizing effects and currents in such a strip can be used toorient the magnetizations antiparallel, and so that externally appliedmagnetic fields can orient the magnetizations parallel. Thus, paralleland antiparallel magnetizations can be established in the ferromagneticthin-films of the structure as desired in a particular use. Such astructure must be fabricated so that any ferromagnetic orantiferromagnetic coupling between separated ferromagnetic films is nottoo strong so as to prevent such establishments of film magnetizationsusing practical interconnection arrangements.

A broader understanding of the giant magnetoresistance effect, i.e. thespin valve effect, can be obtained by considering a generalizedmultilayer structure shown in FIG. 1 and ignoring, for simplicity thoughthis is not necessary, the ordinary anisotropic magnetoresistive effect.The structure is typically provided on a semiconductor chip, 10, havingsuitable operating circuitry therein. An electrical insulating layer,11, supports N identical ferromagnetic thin-film conductive layers, eachseparated from an adjacent one by one of N-1 identical nonmagnetic,conductive intermediate layers to form a superlattice structure. Ahighly resistive outer passivation layer, 12, covers this structure, andsuitable electrical interconnections are made to the conductive layersbut not shown. The conductance of this superlattice structure will bethe sum of the conductances of the individual layers which areeffectively electrically in parallel with one another, but the giantmagnetoresistive effect introduces magnetization dependence in theferromagnetic thin-films. In the following, a possible model isdeveloped to an extent as a basis for gaining a better understanding ofthe electrical and magnetic behavior of this structure, but this modelis simplified by approximations and not all would agree with everyaspect of the approach chosen.

The conductance of very thin-films is highly dependent on surfacescattering if the mean-free path of conduction electrons in the bulkmaterial of the films is equal to or longer than the thickness of thefilms. The ratio of the film conductivity to the conductivity of thefilm material in bulk can be expressed as a function of the ratio of thefilm thickness to the mean-free path of electrons in bulk materialidentical to the film material by the well known Fuchs-Sondheimerconduction model assuming inelastic scattering at the film surfaces, orby other associated models taking further conditions into account suchas grain boundary scattering and other surface scatterings.

The magnetization dependence in the ferromagnetic thin-films leading tothe giant magnetoresistive effect appears dependent on the ratio of spinup to spin down electrons in the 3D shell of the transition elementsused in the ferromagnetic thin-films, i.e. the spin polarization P ofthe conduction electrons. The fraction f of 3D electrons which are spinup have typical values of 0.75 for iron, 0.64 for cobalt and 0.56 fornickel. Conduction electrons in metals are normally S shell electronswhich theoretically would be equally divided between spin up and spindown electrons. However, because of band splitting the conductionelectrons in the magnetic layers are assumed to have a fraction of spinup electrons like that of the electrons in the 3D shell. The spinpolarization is then determined from P=2f-1. Such electrons are assumedin encounters with atomically perfect boundaries between the magneticlayers, including in this boundary the thin nonmagnetic, conductiveintermediate layer therebetween, to be either scattered inelastically orpass freely into the next magnetic layer.

In view of the observed spin polarization, the simplifying assumption ismade that the probability of a spin up electron not being scattered inentering a material having a majority of spin up electrons isapproximately equal to the fraction of the electrons in the conductionband which are spin up, and that the probability of a spin down electrongoing into the same material not being scattered is equal to thefraction of the electrons in the conduction band which are spin down.Changing the magnetization directions between parallel and antiparallelin adjacent ferromagnetic thin-films changes the conduction bandelectrons in the films from having matching spin up and spin downfractions in each to having opposite spin up and spin down fractions ineach. Thus, a larger fraction of the electrons in the superlatticestructure will be scattered when the magnetizations in the ferromagneticthin-films are antiparallel as compared to when they are parallel, sincemore than half of the electrons in the conduction band are spin up inview of the spin up fraction values given above. If the ferromagneticthin-films are separated by a conductor layer which preserves the spinof the conduction electrons in passing therethrough, some conductionelectrons can pass from one layer to the other without collisions and socan travel through effectively a thicker layer than those which arescattered to thereby be confined within a single layer. As a result, thescattered electrons can have a significantly lower conductivity and so,if the ferromagnetic films are oppositely magnetized, there will be agreater effective resistance in the structure. This view of theconduction electron transport between ferromagnetic thin-film layers canbe adjusted for imperfections at the boundaries between adjacentferromagnetic thin-films for conduction band electrons, which would notbe scattered because of the spin thereof, may instead be scattered byphysical imperfections at the boundary.

Based on the foregoing, the effective conductivities for parallel andantiparallel magnetization states in the superlattice structure can bedetermined, and subtracted from one another to provide the ratio ofchange in effective conductivities of the ferromagnetic thin-films, dueto a corresponding change between parallel and antiparallelmagnetizations in those films, to the average conductivity in thosefilms. The result of this determination must have added to it theconductivities of the nonmagnetic, conductive intermediate layers on thebasis of those layers having equal populations of spin up and spin downconduction band electrons, and a conductivity which does not change withmagnetization directions. In such a setting, the ratio of the differencein sheet conductances of the superlattice structure when theferromagnetic thin-films change magnetization from parallel toantiparallel, Δγ.sub.⊥→∥, to the average of these sheet conductances,γ.sub.⊥→∥, can be obtained as ##EQU1## ignoring the ordinary anisotropicmagnetoresistance in obtaining this giant magnetoresistive response asindicated above. Here q represents physical boundary imperfections, andis the probability that a conduction electron which would not bescattered because of its spin is also not scattered by physicalimperfections or collisions in the nonmagnetic, conductive intermediatelayers.

The symbol γ_(m1), is the sheet conductance of a single ferromagneticthin-film, the sheet conductance per unit square of a thin-film beingthe conductivity thereof multiplied by the thickness thereof. Thus,Nγ_(m1) is the sheet conductance of N parallel ferromagnetic thin-films.The symbol γ_(mN) is the sheet conductance of a layer of ferromagneticthin-film N times the thickness of a single ferromagnetic thin-film, andγ_(c1) is the sheet conductance of a nonmagnetic, conductiveintermediate layer.

The number N of ferromagnetic thin-films affects the differences insheet conductances because of the difference in conductivity between aferromagnetic thin-film which is N layers thick compared to Nferromagnetic thin-films electrically connected in parallel. Thepolarization factor P is, as indicated above, expected to be importantin the giant magnetoresistive response in representing the fraction ofspin up conduction band electrons, and this expectation is borne out bythe square of that factor appearing in the numerator of the equationabove.

The quality of the interface between the ferromagnetic thin-films andthe nonmagnetic, conductive intermediate layers is important asrepresented in the last equation by the symbol q. The largest giantmagnetoresistive effect values have been obtained in material systemswhere both the lattice constant and the crystal and the form in thecrystal class of each interface material have been well matched. Forexample, chromium matches the body-centered cubic structure of ironbetter than all other natural body-centered cubic nonmagnetic metals.Copper is similarly the best match for face-centered cubic cobalt andfor face-centered permalloy mixtures which are quite similar to nickel.Significant mismatches will likely give a very low value for q.

Also, scattering in the nomnagnetic, conductive intermediate layers islikely if the thickness of those layers is smaller than the mean-freepath of conduction electrons in the bulk film material. Hence, thesymbol q will be reduced in value for thicker intermediate films.

The film thickness also has a significant effect on the ratio of γ_(mN)/γ_(m1) with this ratio increasing as the films get thinner, as shown bythe Fuchs-Sondheimer conduction model. The greatest conductivitydifference between parallel and antiparallel magnetizations in theferromagnetic thin-films can be seen, from the last expression above, tooccur in the very thinnest of magnetic layers were it not for thescattering and shunting effects of the nonmagnetic, conductiveintermediate layers. However, once the conductance of the magneticlayers, decreasing in being made thinner, gets to be on the order of theconductance of the nonmagnetic, conductive layers, the expression aboveshows that further decreases in thickness will reduce the giantmagnetoresistive effect. Thus, for a fixed set of parameters for thenonmagnetic, conductive intermediate layer, the giant magnetoresistiveeffect will have a peak in value at some ferromagnetic thin-filmthickness.

This assumes that the coupling between the structure ferromagneticthin-films is also arranged to result in an operable device since itdetermines the range of magnetization angles which can be achieved in adevice for given values of applied magnetic fields, so sets limits onthe magnetoresistive response. If positive, or ferromagnetic, couplingis present and too great, the film magnetizations will not besufficiently close to being antiparallel, and perhaps cannot be made soby passing a sensing current through the structure, so that the maximumresistance expected for the configuration cannot be obtained. On theother hand, if negative, or antiferromagnetic, coupling is present andtoo great, the film magnetizations will not be sufficiently close tobeing parallel, and perhaps cannot be made so by applying an externalmagnetic field to the structure, so that the minimum resistance expectedfor the configuration cannot be obtained.

Further, there is a limit on the thinness of the nonmagnetic, conductiveintermediate layer because of "pin holes" occurring therethrough whichresult in that layer covering less than 100% of the surfaces of theferromagnetic thin-films on either side thereof. These "pin holes" inthe nonmagnetic, conductive intermediate layers are thought to lead to acurrent density dependence in the giant magnetoresistive effect which isnot reflected in the last expression above. Such pin holes in thisintermediate layer appear to result in ferromagnetic coupling betweenthe ferromagnetic thin-films on either side of that layer in thevicinity of such holes thereby creating ferromagnetically coupledmagnetic domains in these ferromagnetic thin-films which are otherwiseantiferromagnetically coupled (assuming no external magnetic fieldsbeing applied).

As a result, there appears to be an incomplete saturation ofmagnetizations across the superlattice along the easy axes so thathigher currents through the superlattice structure generate a"scissoring" magnetic field (a field forcing magnetizations in filmsadjacent an intermediate layer in opposite directions) which counteractsthe effects of the pin holes by forcing the magnetizations in the pinhole domains to more closely align with the magnetizations in the restof the ferromagnetic thin-film in which they occur. Sufficiently highcurrents can leave a single domain in each such ferromagnetic thin-film.

Although the effect of a very low pin hole density can be perhapscorrected for "sandwich" structures with two magnetic layers byproviding a sensing current of a sufficient current density through thesuperlattice structure, a relatively small increase in pin hole densitywill quickly lead to all of the ferromagnetic thin-films beingferromagnetically coupled so that the magnetizations therein are in, ornear to being in, a common direction. Such a result will make thesuperlattice structure inoperable as a device, and so there is a desireto provide thin nonmagnetic, conductive intermediate layers with reducedpin hole densities. Further, such layers are desired to be stable inbehavior over an extended range of temperatures. In addition, theresulting devices when for use as magnetic field sensors are desired tohave a sensing characteristic over a range of magnetic fields which issubstantially linear with little hysteresis and high sensitivity.

SUMMARY OF THE INVENTION

The present invention provides a magnetoresistive layered structurehaving a pair of magnetoresistive, anisotropic ferromagnetic thin-filmsseparated by an intermediate layer on a substrate, and perhapsadditional such ferromagnetic thin-films each separated from another bycorresponding further intermediate layers. Each such intermediate layeris less than 50 Å thick and formed of a substantially nonmagnetic,conductive alloy having two immiscible components therein. A furthercomponent to provide temperature stability in some circumstances is tobe at least partially miscible in the first two components. Thethin-films may be two-strata or three-strata films each having a greatermagnetic moment stratum against an intermediate layer. The resultingdevices can be used as magnetic field sensors in a circuit arranged tohave at least one positioned in or near a gap between two permeablemasses and at least one other positioned adjacent to another side of oneof these masses other than one facing the gap or substantially parallelto such a facing side. This later mass can be divided into two masseswith the device positioned adjacent the side of that divided massfarthest from the gap.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a portion of a device knownin the prior art,

FIG. 2 is a diagrammatic representation of a portion of a deviceembodying the present invention,

FIG. 3 is a graph of a characteristic of a device embodying the presentinvention,

FIG. 4 is a diagrammatic representation of a portion of a deviceembodying the present invention,

FIG. 5 is a graph of a characteristic of a series of devices embodyingthe present invention,

FIGS. 6A and 6B are schematic diagrams of circuits useable with thepresent invention,

FIGS. 7A and 7B are schematic positioning diagrams useable with thepresent invention,

FIGS. 8A and 8B are graphs of characteristics of a device embodying thepresent invention,

FIGS. 9A and 9B are diagrammatic representations of a device embodyingthe present invention,

FIG. 10 is a diagrammatic representation of an alternative for a portionof the device shown in FIGS. 9A and 9B, and

FIG. 11 is a diagrammatic representation of an alternate deviceembodying the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows in diagrammatic form a cross sectional view of a portion ofa superlattice device formed in a monolithic integrated circuit chip, oron a ceramic substrate, or on other suitable material. Here, a"sandwich" structure with just two ferromagnetic thin-films with generaldesignations F'₁ and F'₂ is shown separated by an intermediate layer I'₁as an abbreviated superlattice, but additional alternating layer pairsof intermediate layers and ferromagnetic thin-films could be provided ina larger superlattice structure to increase the giant magnetoresistiveeffect in the same manner as is shown in FIG. 1. FIG. 2 is not to scale,and is not in proportion, for purposes of clarity, which is also true ofFIG. 1.

The integrated circuit chip, or other substrate, has again semiconductormaterial or other suitable material for substrate 10 which, in thesituation of semiconductor material in an integrated circuit chip, hastherein electronic integrated circuits provided for operating theabbreviated superlattice structure device thereon. Such a device couldbe intended for use as a memory cell in a digital memory or as amagnetic sensor, for instance. An electrically insulating layer againdesignated 11 is again provided on substrate 10, the upper surface oflayer 11 in the device portion shown supporting the abbreviatedsuperlattice structure indicated above. Insulating layer 11 is shownhere as two separate layers, a lower layer, 11', formed of silicondioxide perhaps 2,000 to 6,000 Å thick, and an upper layer, 11", formedof silicon nitride having a typical thickness of 100 to 1,000 Å. Layers11' and 11" are typically provided in a well-known sputter depositionstep. Use of nitride layer 11" prevents oxygen atoms from diffusing intothe layer to be provided thereon which could chemically attack thatlayer.

The silicon nitride in layer 11" provides an amorphous insulator, thatis, an insulator without any periodic structure typical ofcrystallinity, or, in other words, no long range atomic or molecularorder. Alternatively, layer 11" can be formed of a material which hasthe same form in the same crystallinity class that the next succeedinglayer has to provide a possibly better match between these twomaterials. Since a permalloy material will be used in the succeedinglayer in a face-centered cubic structure, an alternative material wouldbe magnesium oxide which also has a cubic structure and a compatiblelattice constant.

The next process step in forming the structure of FIG. 2 is the sputterdeposition of 40 to 50 Å of a permalloy material comprising 20% cobalt,15% iron and 65% nickel in the presence of an external magnetic field inthe plane of the film oriented along a perpendicular to the view shownin that figure which, as just indicated, results in a film having aface-centered cubic structure. Also, the composition of this mixture informing a permalloy material leaves the layer with approximately zeromagnetorestriction, and the fabrication magnetic field leaves the filmeasy axis along the perpendicular followed by the fabrication field. Themagnetic moment of this material is typically about 12,000 gauss. Thisis designated in FIG. 2 as component F'₁₋₁ of film F'₁, and is the firststratum in two strata first ferromagnetic thin-film F'₁.

A second stratum, F'₁₋₂, is provided in a sputter deposition step,performed in the presence of a similar fabrication magnetic field, as amaterial formed from 95% cobalt and 5% iron to a thickness of at least10 Å to get the full giant magnetoresistive effect, and typically of 15Å thickness. The magnetic moment of this material is 16,000 gauss, avalue higher than that of the magnetic moment of first stratum F'₁₋₁ infirst ferromagnetic thin-film F'₁. Since, as a general matter, thepolarization of a material and the magnetic moment thereof each tend toincrease when the other does, a higher magnetization moment material inthe structure ferromagnetic films gives a larger giant magnetoresistiveresponse, as can be seen from the expression above where the square ofthe polarization appears in the numerator. Thus, having a ferromagneticstratum with the greater magnetic moment nearest the intermediate layerwill typically result in a larger giant magnetoresistive response.

On the other hand, rotating the film magnetization to change itsdirection in second stratum F'₁₋₂ by an external magnetic field duringoperation is more difficult in this harder magnetic material. As aresult, providing first stratum F'₁₋₁ as a permalloy material which is asorter magnetic material than that of the second stratum, the magnitudesrequired of external magnetic fields in operating the device will bekept at values which do not require too large a current to flow in theinterconnections of the device. Hence, a two strata ferromagneticthin-film provides a more optimal ferromagnetic thin-film for asuperlattice structure intended to provide a giant magnetoresistiveeffect response in a monolithic integrated circuit chip. Also, themagnetostriction for second stratum F'₁₋₂ remains near zero so thatfirst ferromagnetic thin-film F'₁ will not have undue forces arisingbetween that film and the substrate below, or the film on the other sidethereof; in the presence of magnetic fields which could lead to achanging of the film material magnetic properties under the resultingstress or even, at some point, to the possibility of a mechanicalfailure of the device.

As indicated above, the provision of an intermediate layer, typicallycopper, in thicknesses below 30 Å leads to increasing pin hole densityas the thickness of such a layer diminishes. The magnetic coupling fieldbegins to rise into the tens of oersteds as this copper thickness goesbelow this 25 to 30 Å value (there is significant variation withdifferent deposition parameter values in the sputter deposition of thatlayer). Yet, as indicated above, the thickness of the intermediate layermust be reduced as the thickness of the ferromagnetic thin-films isreduced to increase the potential for a larger giant magnetoresistiveeffect. The thickness of the nonmagnetic, conductive intermediate layeris reduced so that its shunting effect does not become too great withrespect to the decrease in conductance of the ferromagnetic thin-filmsas they are reduced in thickness. Hence, a measure to avoid anincreasing pin hole density as the intermediate layer is reduced willprovide a basis for achieving a higher giant magnetoresistive effect.

Thus, an intermediate layer, I'₁, is provided in the form of an eutecticalloy through including therein two metals as components of the eutecticalloy which are substantially immiscible with one another so thatpinholes forming in the larger fraction component are "filled in" to asubstantial degree by the smaller fraction component. Since theface-centered cubic crystal structure of copper, and its latticeconstant, are well matched with that of second stratum F'₁₋₂, asdescribed above, the basic component choice of copper is made for theeutectic alloy to form the intermediate layer. A second component ischosen to be silver which has a face-centered cubic structure likecopper although it has a slightly larger lattice constant. Silver andcopper are only slightly soluble in one another in forming solidsolutions, a few tenths of a percent, and form a eutectic alloy having aconductivity which is about 2 to 3 μΩ-cm in bulk which value issubstantially maintained over a wide range of relative composition ofthese two metals in the resulting eutectic alloy (resistivities of thisalloy being a few to several times greater when used to form a thin-filmsuch as that which is to be used to form the intermediate layer).

Though copper is quite able to enter into solid solution with nickel, itwill not do so with cobalt or iron, and silver is substantiallyimmiscible with any of these materials also. Hence, a very thin film ofa copper and silver eutectic alloy can be formed as a very thinintermediate layer on second stratum F'₁₋₂ formed of cobalt and iron, anintermediate layer in which a silver-rich phase material "fills in" thepinholes which occur in the copper-rich phase material constituting thebulk of thin intermediate layer I'₁. The typical thickness used for suchan eutectic alloy film as intermediate layer I'₁ is on the order of 14to 17 Å, with typically 15 Å being chosen.

In an intermediate layer formed of copper alone provided by sputterdeposition, there will be strong ferromagnetic coupling between theferromagnetic thin-films it separates, whereas the addition to a film ofthe same thickness of silver from approximately 15% to 28% atomic in thesputter deposition process will result in antiferromagnetic coupling ofa relatively low value. Such a result can be extended to a superlatticestructure of a larger number of alternating intermediate andferromagnetic thin-film layers than shown in FIG. 2 with a similarresult.

The relative amount of silver in the film has been kept as small aspossible in achieving such antiferromagnetic coupling both becausesilver by itself is known to lead to strong ferromagnetic coupling, andbecause the lattice mismatch of silver with first ferromagneticthin-film second stratum F'₁₋₂ on each side thereof is significantlylarger than is that of copper. Nevertheless, keeping the relativefraction of silver to less than 30% results in the reduction of thegiant magnetoresistive response due thereto, because of latticemismatch, still being substantially less than the gains in the giantmagnetoresistive effect due to lowered or eliminated ferromagneticcoupling because of the filled pinholes. In some situations, evengreater fractions of silver are expected to be suitable.

Thus, intermediate layer I'₁ is formed by sputter deposition using acopper sputtering target. That copper target is supplemented by theaddition of silver tabs thereto to the extent needed to result in thedesired silver relative fraction of the resulting eutectic alloy, afraction typically chosen to be 23% to give the desired result. Aseparate silver sputtering deposition target could be used with a coppertarget or, alternatively, a copper-silver eutectic alloy target could beused having the same composition as is desired for films depositedtherefrom. The intermediate layer from such a deposition step is ametastable face-centered cubic structure which typically remains stablefor a long duration if the device is kept within expected temperatureranges during its experience thereafter.

As an alternative to the binary alloy layer formed as an eutectic alloywith grains of silver among grains of copper, gold may be used with thesilver and the copper to provide a ternary alloy to serve asintermediate layer I'₁. Although a binary alloy layer formed of copperand silver as described above forms an excellent intermediate layer inproviding antiferromagnetic coupling between the ferromagnetic layersthat are to be provided on either side thereof for the proper thicknessselection thereof, the ability of that intermediate layer to providesuch benefits has been found to deteriorate at temperaturessignificantly higher than room temperature, in some instances occurringin just a matter of hours when the temperature reached the vicinity of150° C. The addition of 5% to 10% atomic of gold to form a ternary alloysystem has been found to greatly improve the temperature stability ofthe magnetic coupling at significantly higher temperatures, i.e. leavingthe coupling substantially unaffected until the temperature reaches 200°C. to 250° C.

In one such ternary alloy used in forming this intermediate layer, theintermediate layer by weight was 75% copper, 15% silver and 10% gold tothus form the CuAgAu ternary alloy layer. This is representative ofsuitable ternary alloy compositions for the intermediate layer in havingthe portion by weight which is not formed by the gold being split so asto be approximately 80% copper and 20% silver. Providing such a ternaryalloy layer by sputter deposition requires adding gold tabs as well assilver tabs to the copper sputter deposition target so that thesematerials are present in the final target in the proportion desired foreach, or the addition of appropriate separate silver and gold targets tothe copper target. Alternatively, a ternary alloy of these materialscould form a single sputtering deposition target having the proportionsof each component therein matching those desired for the layer resultingfrom the deposition. Again, the result of the sputtering deposition is alayer with a face-centered cubic structure.

The success of such a ternary alloy being used as the intermediate layercomes about for reasons that are not fully known or verified. Althoughcopper and silver are substantially immiscible so that the silverapparently fills in what otherwise might be pinholes in the copper,there is the possibility that at elevated temperatures some of thecopper and silver grains in the alloy thereof grow as others shrink.This may result in a structure with significantly larger grains fromeither draining some, or permitting the occurrence of additional,pinholes, and may form a rougher surface for the intermediate layer.Such addition of pinholes or surface roughness, or both, can severelyaffect the coupling between the ferromagnetic layers on either side ofthe intermediate layer. The addition of gold which is miscible with bothcopper and silver to some degree may form binary alloys at theboundaries of these grains within the ternary alloy layer which impedegrain growth to thereby stabilize the layer structure.

The provision of intermediate layer I'₁ is followed by providing asecond two-strata ferromagnetic thin-film, F'₂. A lower stratum for thisfilm, F'₂₋₂, is deposited in the same manner as, with the result thereofbeing substantially the same as, upper stratum F'₁₋₂ of firstferromagnetic film F'₁. Similarly, an upper stratum, F'₂₋₁, of secondferromagnetic film F'₂ is deposited in the same manner as, with theresult thereof being substantially the same as, lower stratum F'₁₋₁ offirst ferromagnetic film F'₁. Finally, a tantalum nitride layer, 12, isprovided on second ferromagnetic film F'₂ as a passivation andprotection layer. After a completion of passivation layer 12, ionmilling is used to form the abbreviated superlattice structure in theform of a strip supported on layer 11". A tungsten interconnection, 13,is shown formed across the end of the abbreviated superlattice structureand onto the upper surface of layer 12.

Thus, the abbreviated superlattice of FIG. 2 providesantiferromagnetically coupled ferromagnetic thin-film layers separatedby a nonmagnetic, conductive intermediate layer. This structure hasprovided coupling fields of 10 to 15 Oersteds, well within the desiredrange of zero to 50 Oersteds to permit relatively easy manipulations ofthe magnetizations in the ferromagnetic thin-films of the structure byexternal magnetic fields. Thus, the magnetizations in each offerromagnetic thin-films F'₁ and F'₂ are stable when directedantiparallel to one another in and out of the plane of the paper onwhich is shown FIG. 2, i.e. along the easy axis of the structure in thatfigure.

In this last magnetization condition, there is no anisotropicmagnetoresistive effect contribution to the resistance of the devicesince any sensing current passing therethrough from interconnection 13on the end shown in FIG. 2 to a similar interconnection on the oppositeend of the structure, not shown in that figure, is perpendicular to themagnetizations in both ferromagnetic thin-films. The giantmagnetoresistive effect gives a maximum resistance in this conditionsince the magnetizations of the two ferromagnetic thin-films F'₁ andF'₂, as stated above, have antiparallel magnetizations.

If an external magnetic field is applied directed into the plane of thepaper on which FIG. 2 is presented, the anisotropic magnetoresistiveeffect contribution to the resistance of the device is unchanged since asufficiently strong external field leaves the film magnetizationsparallel to one another. However, assuming the external field issufficient to rotate the magnetization in the opposite direction inwhich it was oriented prior to the application of the field, as occursfor the magnetization of one of the ferromagnetic thin-films, the giantmagnetoresistive effect contribution to the resistance will be minimizedsince the magnetizations are then parallel to one another each orientedin the same direction.

If, in the alternative, the external magnetic field is applied parallelto the strip, and so along the plane of the paper in which FIG. 2 isprovided, the giant magnetoresistive effect contribution to theresistance will remain a minimum since the magnetization in each offerromagnetic thin-films F'₁ and F'₂ will be rotated to point in thelong direction of the strip and so be parallel to one another. On theother hand, anisotropic magnetoresistance effect will make its maximumcontribution to the resistance since the magnetizations in eachferromagnetic thin-film, in being parallel to the long direction of thestrip, are also parallel to any sensing current being conducted betweeninterconnection 13 and its counterpart at the opposite end of the strip.

Such external magnetic fields have been assumed sufficiently large inthe foregoing to force the magnetizations in ferromagnetic thin-filmsF'₁ and F'₂ to become aligned with the direction of that field. This maybe appropriate for use in a magnetoresistive state sensing memoryserving as digital memory. Alternatively, the external fields may besuch that they cause only partial rotations in the magnetizationsallowing the abbreviated superlattice described, or a more extensivesuperlattice, to serve as a magnetic field sensor.

The abbreviated superlattice structure described above with a ternaryalloy forming the intermediate layer, or a more extensive superlatticebased on adding timer layers to that structure that are repeats oflayers I'₁ and F'₂, can provide very near to ideal characteristics for amagnetic field sensor. Such a structure can substantially avoid theproblems found to occur with the use of other materials in such asuperlattice structure for providing a giant magnetoresistive response,for instance, the use of copper alone to form the intermediate layer orlayers. The use of other kinds of materials in such a structure leads tostructures which typically exhibit relatively high magnetizationsaturation fields that can lead to insufficient sensitivity in theabsence of countering measures, exhibit considerable hysteresis inresponse to changing exterior magnetic fields, and exhibit a non-linearresistance-magnetic field characteristic often requiring a magnetic biasfield to achieve reasonable linearity of the output signal versus theinput magnetic field.

A resistance versus applied magnetic field characteristic (scaled froman unpatterned water) is shown in FIG. 3 for a magnetoresistive basedmagnetic field sensor of the kind described above using the alloymaterials described above for the nonmagnetic layers therein. Thischaracteristic of the sensor exhibits a maximum resistance changebetween a zero applied field and a field sufficiently great to reach theminimum resistance possible as a result of the application thereof; i.e.saturation, that is equal to approximately 10% of the total sensorresistance occurring therein for an applied field that provides half, orthe average, of such a resistance change. This resistance change due tothe giant magnetoresistive effect results from the sensing of an appliedmagnetic field by this sensor of a magnitude sufficient to alignmagnetizations of the magnetic layers with the direction of the appliedfield which is usually applied in a direction perpendicular to that orthe easy axis as the direction in which the sensor is most sensitive,though other directions could be used to form a substantially similarcharacteristic.

The sensor having this characteristic was fabricated generally like theone described above, but with the use of essentially two additionalferromagnetic layers and two additional intermediate nonmagnetic layers(two further high magnetic saturation strata are also required inaddition). Also, the intermediate nonmagnetic layers are formed using aternary alloy system to improve stability with temperature, the alloy inthis instance being a mixture of 82% copper, 12% silver and 6% gold usedto provide for this structure a layer thickness of 16.5 Å. Arepresentation of this structure is provided in FIG. 4 in which thedesignations used for the structure layers are a continuation of thoseused in FIG. 2 except for the interconnection layer at the top and theinterior ferromagnetic layers. The interior ferromagnetic layerdesignations differ in that the interior ferromagnetic layers each havethree strata with a central stratum of permalloy type material 40 Åthick as before with a F'_(x-1) designation (using x to represent theferromagnetic layer number which can be either 2 or 3 for interiorlayers), a lower stratum of a material having a high magnetic moment 10Å thick with a F'_(x-2) designation, and an upper stratum of highmagnetic moment material also 10 Å thick with a F'_(x-3) designation.The interconnection layer has been redesignated 13' as it is now formedof tantalum rather than tungsten, and the tantalum nitride layerpreviously used for passivation and protection is no longer present.

As can be seen from FIG. 3, the maximum electrical resistance of thesensor is 9.77 kΩ which occurs in the absence of any substantialmagnetic fields. At saturation, which occurs at approximately 200 Oe,the resistance with a field in either direction drops to approximately8.82 kΩ. The difference between this last resistance and the maximumresistance is approximately 950Ω the maximum possible resistance change.

As can also be seen, the characteristic extending from maximumresistance value (at which the value of the applied field is zero) looksmuch like the equal sides of an isosceles triangle in that theresistance decreases approximately linearly with applied magnetic fieldin either applied magnetic field direction. Thus, the magnetoresistivecharacteristic of the structure is in a region of linear operationessentially from the zero field value until nearly reaching saturationin either applied field direction. Hence, there is no requirement toprovide a magnetic field bias to the structure for measuring themagnitude of the applied field to enable the sensing to occur in alinear region of the resistance versus applied field characteristic ofthat structure.

In addition, the characteristic of FIG. 3 is made following a cycle ofchange in the applied magnetic field, that field going from zero to oneof the saturation values and then through zero again to the othersaturation value, and then back to zero. As can be seen, there is verylittle hysteresis in the resistance characteristic over a cycle of themagnetic field from one saturation value to the opposite saturationvalue and back. As a result, this resistance characteristic isessentially a single-valued function of the applied field whichessentially removes the possibility of an unwanted sudden transitionoccurring from one resistance value to another at a given fieldmagnitude.

In forming a sensor with such a structure, the magnitude relationshipbetween the magnetic field to be measured and the saturation field forthe structure must be considered. If the saturation field of thestructure is significantly larger than the applied field to be measured,only a fraction of the possible magnetoresistance change due to thefield being measured is usable in the sensor. Thus, there may bedifficulty in resolving differences between values of magnitude offields which are relatively small compared to the saturation field ofthe structure.

This problem is compounded by the desire to have a relatively largemagnetoresistive response, i.e. a giant magnetoresistive response, inthe structure to provide an easily used output signal. This is becauselarger giant magnetoresistive effects in such sensor structures tend tocorrelate with greater saturation fields characterizing thosestructures, that is, a relatively large applied magnetic field is neededto saturate the structure resistance change by forcing the initiallyantiparallel magnetizations in the antiferromagetically coupled magneticlayers to parallel one another.

In this regard, interlayer antiferromagnetic coupling betweenferromagnetic layers, each of thickness T_(m) and saturationmagnetization M_(s), across a conductive nonmagnetic layer is thought tobe related to phenomena at the interfaces between nonmagnetic andferromagnetic layers, i.e. at the abutting surfaces of adjacent layerseach of surface area S. The strength of the resulting surface couplingenergy is higher for higher saturation magnetization materials (thoughtto be proportional to the square of the saturation magnetization), butdecreases exponentially with increases in thickness T_(c) of thenonmagnetic layers (although oscillating between ferromagnetic andantiferromagnetic coupling over a range of thicknesses). An externallyapplied magnetic field in one direction along the easy axes of theferromagnetic layers can at some magnitude force the magnetizations ofthese layers to parallel one another along the applied field direction.At that point, the change in coupling energy and the magnetostaticenergy of the applied field equal one another. Choosing E_(i) asrepresenting the surface area energy density difference between paralleland antiparallel orientations of the ferromagnetic layers magnetizationswith the effective coupling field (a threshold that the external appliedfield must reach to reverse the magnetization in the layer which isantiparallel to that applied field) taken as H_(coupl), this energybalance can be approximately written M_(s) ST_(m) 2H_(coupl) =E_(i) S sothat H_(coupl) is found to be E_(i) /2M_(s) T_(m).

If another nonmagnetic layer and another ferromagnetic layer like layersI'₁ and F'₂ are added onto layer F'₂, the lowest energy magnetizationconfiguration in the presence of a small externally applied field is forthe two outer ferromagnetic layers to be magnetized in the direction ofthat field with the interior ferromagnetic layer magnetized in theopposite direction. Thus, in this situation, the interior ferromagneticlayer will have a potential energy due to the externally applied fieldthat is twice the value found for the structure described above sincethere are now two surfaces resisting the opposite directionreorientation of its magnetization, or H_(coupl) =E_(i) /M_(s) T_(m).

Thus, the coupling field is found to increase with an increase in thenumber of ferromagnetic layers present in the sensor structure. There isdoubt as to whether further coupling field increases occur or not forfurther additions of nonmagnetic and ferromagnetic layers, there beingsome thought that increases occur by a factor of (1-1/N) with N beingthe number of ferromagnetic layers in the structure. In any event, thecoupling field increases to at least some degree with the number offerromagnetic layers and it increases with the saturation magnetizationdespite Ms appearing in the denominator of the last two equations sinceE_(i) is proportional to the square of the saturation magnetization asindicated above. On the other hand, the coupling field will decreasewith increasing thickness of the nonmagnetic layer or layers since E_(i)decreases exponentially therewith. Increasing thicknesses of theferromagnetic layers will also reduce the coupling field as can be seenby the presence of the factor T_(m) in the denominator of the last twoequations.

Consider now the magnitude of the giant magnetoresistive effect givenabove as ##EQU2## which can be rewritten by noting that the sheetconductances therein can be converted to corresponding layerconductivities through division of these conductances by thecorresponding layer thicknesses. That is, substituting for the sheetconductances in the last equation on the basis of σ_(mN) =γ_(mN) /T_(m),σ_(m1) =γ_(m1) /T_(m) and σ_(c) =γ_(c) /T_(c) yields ##EQU3## Since P,the polarization of the conduction electrons in the ferromagneticmaterial is thought to be proportional to the saturation magnetization,the giant magnetoresistive response also increases with increasingsaturation magnetization values. That effect also increases withincreases in the number of ferromagnetic layers, and with a decreasingthickness of the nonmagnetic layer as can be seen from this lastequation. Despite the factor representing the thickness of theferromagnetic layers which occurs beneath the line in the fractionforming the term on the right in the denominator of the above equationsuggesting that the giant magnetoresistive effect decreases withdecreases in the value of this factor, the opposite is in fact true.Instead, the giant magnetoresistive effect increases with decreases inthis thickness because the numerator factor (σ_(mN) -σ_(m1)) increaseseven more with decreases in ferromagnetic layer thickness since thiseffect is thought to depend on surface scattering and such scatteringincreases with respect to bulk scattering as the ferromagnetic layer isthinned.

Hence, the same parameter changes which lead to higher values for thegiant magnetoresistive effect also lead to higher values for thecoupling field, and so the high correlation therebetween. As a result,superlattice or sandwich structures designed for high values of thegiant magnetoresistive effect will also tend to have high couplingfields leading to relatively large applied field values being needed toreach saturation values of the resistance change present in thestructure due to the magnetoresistivity thereof. Thus, a measure isdesired that will allow a structure having a large giantmagnetoresistive effect to also respond over the much of the possibleresistance range change therein to changes occurring in relatively smallmagnitude magnetic fields in those situations in which such fields areto be sensed.

A mass of magnetically permeable material, especially one which isrelatively long compared to its width which is positioned so that itslength is aligned with the field component of interest, can concentrateseveral times the magnetic flux emanating from those surfaces thereoforiented substantially perpendicular to this component over the ambientvalue of this field component. Such a flux concentrator will thusconcentrate the ambient flux in which it is positioned to effectively"amplify" the ambient flux and provide the resulting increase in flux towhatever object is more or less adjacent to such an end. If that objectis chosen to be the giant magnetoresistive effect structure describedjust above, a relatively low ambient field which would be subject tochanges therein that would make use of only a relatively small portionof the resistance change available in the structure due to itsmagnetoresistivity would, by a proper concentrator design, besufficiently "amplified" so as to make use of nearly all of thatavailable resistance change in that structure. This would give muchbetter sensitivity for that structure despite it having a highresistance saturation field value as is characteristic of the largegiant magnetoresistive effect exhibited thereby.

An approximation can be made of the concentration factor for a pair ofsuch flux concentrators where each is formed as a rectangular solid oflength L, width W and thickness T, and located with respect to the otherso as to have the lengths thereof parallel a common axis but having agap of length G between them in or near which one or more giantmagnetoresistive effect structures of the kinds described above can bepositioned. The approximation is based on considering each of thesesolids as a prolate spheroid having a maximum cross sectional area,perpendicular to its major axis, equal to the cross sectional area ofits corresponding rectangular solid perpendicular to its length, andhaving the length of its major axis equal to the length of itscorresponding solid. Any magnetic interaction between a fluxconcentrator and the other, or with any other magnetic materialstructure, is ignored as being negligible.

The internal magnetic field, H_(in), in such a mass of magnetic materialis the difference between the applied field, H_(a), and thedemagnetizing field resulting therein, H_(D), or H_(in) =H_(a) -H_(D).Since the demagnetizing field is defined as H_(D) =4πNM where N is thedemagnetizing factor for the body of magnetic material involved and M isthe magnetization thereof which is substantially constant for a prolatespheroid, the internal field becomes H_(in) =H_(a) -4πNM. The magneticflux density in the spheroid is B_(in) =H_(in) +4πM or, alternatively,B_(in) =μ_(r) H_(in), assuming the flux and internal field are linearlyrelated (not near saturation), where μ_(r), is the reversiblepermeability. Thus, the magnetization can be written from equating theright hand sides of these last two equations as ##EQU4## Substitutingfrom the last equation above for H_(in) preceding the two fluxexpressions yields ##EQU5## Thus, if μ_(r) is sufficiently large and themagnetic material mass is in the shape of a prolate spheroid with thelength of the major axis thereof sufficiently limited to keep N frombeing too small, M=H_(a) /4πN.

A magnetized prolate spheroid is known to be capable of having itscontribution to the exterior magnetic field from the magnetizationthereof characterized from an effective surface distribution of freemagnetic poles. In determining the contribution thereof to the exteriorfield along the axis extending into the gap from the major axis thereof(this major axis being coincident with the spheroid x axis having x=0 atthe center of the spheroid, the y axis being along the vertical minoraxis thereof with the same origin), an equivalent line pole distributioncan be found along the spheroid major axis by finding the change in areaof the cross section of the spheroid perpendicular to the major axisthereof for changes in position along this axis multiplied by theapproximate magnetization found above. This cross section is a circle ofa radius which changes with position along the major axis reaching amaximum value of b on a minor axis at the middle of the spheroid, andhas an area, A, which can be written πy² so that its change with xbecomes ##EQU6##

The cross section of the prolate spheroid through both the major axis (xaxis) thereof and the minor axis that is also the y axis (a crosssection view that is perpendicular to the circular cross section justdescribed) is an ellipse with a minor axis of length 2b=2(TW/π)^(1/2)and a major axis of length 2a=L which can be described by the equation##EQU7## from which ##EQU8## Noting that the magnetic pole linedistribution, p(x), is equal to -MdA/dx since there is a pole densityincrease toward the major axis ends of the prolate spheroid in the poledistribution towards which the circular cross section area decreases,the pole line distribution is ##EQU9##

From electromagnetics theory, the field in the gap due to one of themagnetized prolate spheroid magnetic pole line distributions, H_(g:ld),will be given by the equation H_(g:ld) =p(x)/r where r is the distancefront the center of the gap to a magnetic pole in the spheroid given byr=(G/2+a)-x. Two such prolate spheroids, one on either side of the gap,means, in the absence of any magnetic interaction therebetween, that thefield at the center of the gap will be doubled. Taking into accountthese considerations, the magnetic field at the center of the gap,H_(G/2:2ld), is found by integrating the field equation over themagnetic pole distribution of one of the spheroids, or ##EQU10## Thisintegration can be carried out to provide a closed form result of##EQU11##

In addition to the field in the gap due to the magnetized solids thereis also present the applied field H_(a) which must be added thereto toobtain the total field at the center of the gap, H_(G/2:T), or H_(G/2:T)=H_(a) +H_(G/2:2ld). The concentration of the field in the gap due tothe presence of the magnetized solids can be characterized by aconcentration factor, C_(G/2), formed by the ratio of the total gapfield to the applied field, or C_(G/2) =H_(G/2:T) /H_(a). Aftersubstituting the values given above for M, a and b, this factor becomes##EQU12## As is known for prolate spheroids, the demagnetizing factor Ntherefor, for applied fields along the major axis thereof, is ##EQU13##which results in a concentration factor of ##EQU14## Because of theapproximations made, especially the ignoring of the interaction betweenthe two magnetized solids, this result has been found to be low by afactor of approximately two. Nevertheless, the relative importance ofthe various parameters involved in the concentration of the appliedfield is indicated by this result.

The permeability of the magnetic material used in the solids isunimportant if it is sufficiently large, i.e. greater than a thousand,as it then no longer significantly affects the magnetizations thereof ifthe lengths of the concentrator solids are not too great. The thicknessand width of the solids is also relatively unimportant as can be seenfrom the last result although certainly there should be enough magneticmaterial present so that the concentrator magnetic materials do notreach magnetic saturation before any giant magnetoresistive effectstructures positioned in the gap therebetween experience magnetic fieldssufficient to saturate the resistance change due to that effect. Rather,the ratio of the length of each of the solids to the length of the gapis important. This is seen in the graph of FIG. 5 plotting the measuredfield concentrations in the gaps of similar devices to the correspondingconcentrator length-to-gap length ratio. As can be seen, at largerlength ratios the concentration factor is numerically quite similar invalue to corresponding length ratio value.

Thus, the giant magnetoresistive effect structures described above canbe provided with greater sensitivity in sensing smaller applied magneticfield changes by being located in the gap between two concentrators ofsignificant length along an axis passing through each and the gap. Inpractice, more than a single such structure will usually be positionedin the gap to experience and react to concentrated magnetic fields thereas these structures are usually connected in some sort of multiplestructure electrical interconnection arrangement such as a bridgecircuit. Such circuit arrangements are usually provided to bothconveniently provide for the electrical energization of the variousstructures present and for the measuring of signals generated therebyunder the influence of applied external magnetic fields.

As examples, a bridge circuit in which giant magnetoresistive effectstructures are represented as circuit resistors is shown in schematicform in FIG. 6A. A voltage source, 20, is shown connected between twoopposing nodes of that circuit at each of which two of the four suchresistors are electrically connected, the negative side of the sourcebeing shown grounded. In addition, the inverting and noninverting inputsof a signal sensing amplifier, 21, are each electrically connected to anopposite one of the two remaining opposing nodes in the bridge circuitat each of which also two of the four resistors are electricallyconnected. In FIG. 6B, two giant magnetoresistive effect structures areshown as circuit resistors with one end of each being electricallyconnected to ground. A pair of current sources, 20' and 20", each alsohave one end electrically connected to ground, and the opposite endthereof is electrically connected to (a) a corresponding one of theseresistors, and (b) a corresponding one of the inverting and noninvertinginputs of a signal amplifier, 21'.

In each of these figures, the symbols R₁ and R₂ are used to bothdesignate the structures, or resistors, and to represent the electricalresistance values thereof, there being two pairs of equal valueresistors in the FIG. 6A circuit. Typically, the structures serving asthe resistors in either of these circuits are fabricated to match oneanother (although they need not be), and so have substantially identicalresistance values under identical conditions.

In each of these circuit arrangements, adjacent ones of the resistors inthe circuit must vary differently from one another under an appliedmagnetic field if a signal output is to result. If they each have thesame variation, there will be a zero value signal developed between thesensing amplifier inputs. This will be true even if the field directionis reversed for adjacent resistors but of the same magnitude for eachbecause the characteristic shown in FIG. 3 shows the same resistancevalue will still occur for each resistor. Thus, the magnitude of thefields to be sensed by these circuits must differ for adjacent resistorstherein to provide a sensing signal representing those fields. If themagnitudes of the fields, or the changes in the fields, to be sensed aresufficiently large, flux concentrators need not be used with the giantmagnetoresistive effect structures (or resistors) in either of thesecircuit arrangements, but for smaller magnitudes such concentrators canbe used with those structures in either circuit.

The giant magnetoresistive effect structures, or resistors, in either ofthese circuits will be arranged in use so that the lengths thereof willprimarily parallel the direction along which the applied magnetic fieldsare to be sensed, and also parallel the long axes of any fluxconcentrators that are used therewith. If opposite pairs of theresistors of FIG. 6A are sufficiently separated from one another inspace, or if the two resistors in FIG. 6B are sufficiently separatedfrom one another in space, so that different magnitudes of the appliedmagnetic field are present at these separated locations, the outputsignals from the circuits will represent the gradient of the appliedfield. Thus, spatial variation in the magnitude of the applied magneticfield can be used to provide an output signal in these circuits.

However, such spatial variation may not be of interest, or available inthe confines of an integrated circuit in which such circuits areprovided. In such circumstances, design measures must be taken to assurethe desired applied magnetic field magnitude differences at the circuitstructures or resistors. Such measures will be described with respect tothe circuit of FIG. 6A but can be used with the circuit of FIG. 6B ifdesired. The first option to be described is the use of a biasingmagnetic field that biases different structures or resistors used in thecircuit differently.

Thus, in the circuit of FIG. 6A, the opposite pairs of structures orresistors are biased by a locally provided magnetic field in oppositedirections along the desired measurement direction so that an appliedmagnetic field which will be oriented in a single direction willincrease the total field in one of the pairs but decrease the totalfield in the other. Two examples are shown in FIGS. 7A and 7B with thestructures or resistors of the FIG. 6A circuit shown provided on asubstrate, 10',11', so that the two resistors designated R₁ arepositioned as a group on the left side of substrate 10', 11' and so thatthe two resistors designated R₂ are positioned as a group on the rightside of substrate 10',11', with the resistors of each group having theprimary lengths thereof parallel with, and extending toward, those ofthe other. Though not shown, these resistors are interconnected in thecircuit of FIG. 6A.

In FIG. 7A, a permanent bar magnet is placed in plane parallel to theplane in which substrate 10',11' extends with the magnet length axiscentered between the two resistor groups. As a result, the magneticfield emanating from the magnet end nearest the substrate has componentsin it which pass in one direction through the resistors in one of thetwo groups thereof and in the opposite direction through those in theremaining group, these field components passing through the primarylengths of each member of each group. Thus, the resistors in each groupare magnetically biased in a field direction opposite the direction inwhich they are biased in the other group. Alternatively, the permanentbar magnet is positioned underneath substrate 10',11' (shown in dashedlines in FIG. 7B) with its length axis perpendicular to the plane ofextent of that substrate to provide substantially the same result. Ifthe permanent bar magnet is formed of a material to have a fairly largesaturation induction of 5000 Gauss or more, and a large coercive forceof 2000 Oe or more, the bias magnetic field provided thereby will notvary much with the applied field to be sensed.

The corresponding resistance versus applied field characteristics forthe biased resistors of FIGS. 7A or 7B, approximated by two sides of atriangle, are shown for a resistor from each group. The operating pointsdue to the bias, or the bias points, for a bias magnetic field componentof magnitude H_(b) resulting for these resistors are marked on thesecharacteristics in FIG. 8A as darkened dots, but with one shown asnegative bias and the other positive in keeping with the oppositedirection bias field components in each. In FIG. 8B, the result of suchbiased resistors experiencing an applied field H_(a) from a sourceexternal to the systems of FIGS. 7A or 7B that is to be sensed ofmagnitude H_(a-1) is shown. This external applied field will have anycomponent therein along the primary lengths of the resistors in eithergroup be in the same direction for both so that the total field for theR₁ resistors increases to decrease the resistance values thereof, butthe total field decreases for the R₂ resistors to increase theresistance values thereof. The result is to give the new operatingpoints for these resistors marked by darkened dots in FIG. 8B. Reversingthe direction of the sensed field will give the opposite result insteadof increases in resistance value for R₁ resistors and decreases for R₂resistors.

Such arrangements as shown in FIGS. 7A and 7B will probably, because ofthe biasing requirements, be used without the presence of fluxconcentrators that would complicate setting the biasing magnetic fieldalthough not necessarily. In any event, as shown, these arrangementsresult in a sensor that is sensitive to both the magnitude of theapplied magnetic field to be sensed, and to the direction thereof. Thiscan be seen from finding the output signal, v_(s), for such anarrangement serving as an input to the sensing amplifier by noting thatthe two sides of the FIG. 6A circuit across voltage supply 20 are eachvoltage dividers. Using the well known voltage divider equation andtaking the supply voltage to be of value V, the voltages at the centernode of each divider can be found and subtracted to provide this outputsignal, or ##EQU15##

Returning to FIG. 8B, the positive applied magnetic field portion of theR₁ resistor characteristic, involved because of the direction ofmagnetic field bias chosen, can reasonably be represented by a linearfunction with a negative slope between a zero applied field value andapproximately the applied field value saturating the giantmagnetoresistive effect resistance change. Such a function can bewritten as

    R.sub.1 -R.sub.o -K(H.sub.a +H.sub.b),

where R_(o) is the structure resistance value in the absence of anapplied magnetic field, and K is the effective conversion constantbetween the applied field and the resulting resistance value due to thegiant magnetoresistance effect. Similarly, the negative applied magneticfield portion of the R₂ resistor characteristic, again involved becauseof the direction of the magnetic field bias chosen, can reasonably berepresented as a positive slope linear function or

    R.sub.2 =R.sub.o +K(H.sub.a -H.sub.b).

Substituting these last two representations into the expression for thebridge output signal given just preceding them gives the result##EQU16## Thus, the output signal will follow the magnitude of, and itspolarity follow the direction of, the applied magnetic field to besensed at least so long as that field has a magnitude which does notexceed the bias field.

An alternative implementation of the FIG. 6A bridge circuit which alsoprovides differing responses in the bridge circuit resistors, orstructures, to an applied magnetic field to be sensed is to form eitherthe R₁ resistors or the R₂ resistors of a material which does notrespond to such an applied field. A better result for this kind ofimplementation can be obtained by forming all of the bridge circuitstructures, or resistors, as before but shielding either the R₁resistors or the R₂ resistors to prevent them from experiencing theapplied field by positioning a mass of highly permeable material overthem. Then an output signal from the bridge circuit representing theapplied magnetic field can be obtained, and the similar or matchingresponses of each of the structures, or resistors, used in the circuitto operating temperature changes will be balanced against one another.As a result, the response of the bridge circuit in its output signal tosuch temperature changes will be reduced or cancelled.

A shield, to be effective in preventing an applied magnetic field fromsignificantly affecting the resistors on the substrate thereunder, mustnot saturate since the relative permeability thereof must be large toprovide good shielding of regions close thereto. This can be seen fromthe expressions given above for fields in a magnetic material mass forwhich the magnetization away from saturation was found in terms of theinternal field to be ##EQU17## and the internal field was given asH_(in) =H_(a) -4πNM. Eliminating the magnetization between theseexpressions allows the internal field to be found as ##EQU18## theapproximation assuming again that μ_(r) is sufficiently large and thatthe magnetic material mass is in the shape of a prolate spheroid, or arectangular solid that can be approximated by a prolate spheroid, withthe length of the major axis thereof sufficiently limited to keep N frombeing too small.

The tangential magnetic fields at an interface between two dissimilarmaterials are known from electromagnetics to be equal. Thus, since thisinternal field is taken parallel to the major axis of the shieldapproximated as a prolate spheroid, there will be a magnetic field justoutside the top, bottom and side surfaces thereof equal to H_(a)/Nμ_(r). In other words, the field just exterior to these shieldsurfaces is reduced from the value of the applied field by a factor of1/Nμ_(r). Use of the demagnetizing factor given above for a prolatespheroid results in this surface location reduction factor being equalto ##EQU19## Thus, within fabrication process limitations, givenresponsive choices for the other parameters and suitably large relativepermeability of the shied material, the shield thickness can beincreased sufficiently to provide the desired surface location fieldreduction factor value at the surfaces indicated shove.

There is a limit to how closely the resistors being shielded can bepositioned in the substrate next to the shields, however, and thereduction in the field diminishes the further away from the shieldmaterial the resistor position occurs until some separation between themis enough so that the local field there has essentially the value of theapplied field. Thus, there is a remote location field reduction factorthat depends on the geometrical relationship between the shield and theresistor location which diminishes with increasing separation of theshielded resistors from the shield. Approximating a rectangular solidmass of magnetic material used for a shield again as a prolate spheroidand assuming the resistors being shielded are positioned below thecenter of that solid, magnetostatics considerations provide the changein field as a function of the distance into the substrate, z, separatingthe resistors from the shield. Assuming no interactions with othermagnetic materials and with the applied field provided along the majoraxis of the prolate spheroid, the field component parallel to theapplied field at distance z into the substrate is found from thetangential field present at the substrate interface surface equal toH_(in), as found above, multiplied by a geometrical dependent remotelocation field reduction factor, the result of which is then summed withthe applied field. That factor is found to be ##EQU20##

The logarithmic term in the last expression, in having a fractionalargument, has a negative value to result in a term in the full fieldcomponent expression that is subtracted from the applied field toaccount for the shielding effect on resistors located sufficiently nearthe shield. Taking the maximum value of z as -c to evaluate thisexpression, and finding the expression value at proposed shield-resistorseparations, allows forming a ratio giving the fractional change of theincrease in the field in the applied field direction. A limit on thisincrease determines the allowed separation for a given set of length andwidth values for the rectangular solids. Meeting that limit andaccommodating the separation minimum resulting in the fabricationprocess may result in having to adjust the difference between the shieldlength and width to obtain the desired shielding effect.

Although the desired value for the surface location field reductionfactor at the surfaces indicated above may set a minimum thickness limiton the rectangular solid magnetic shield material, a further limit mustbe considered to also satisfy another requirement given above. That is,the shield material must be present in sufficient quantity to preventsaturation thereof by the applied field if the shielding effect is to bemaintained. If the saturation demagnetizing field, H_(D-sat), is greaterthan the maximum applied field, the shield material will not saturate.As indicated above, the demagnetizing field can be represented as H_(D)=4πNM. Using the demagnetizing factor given above for a prolatespheroid, this becomes ##EQU21## Setting M=M_(sat), or to themagnetization saturation value, and using the typical known valuetherefor for permalloy materials of around 10,000 Gauss, allows findingthe saturation demagnetizing field from this expression. Knowing therange of applied fields expected to be sensed, the shield geometricparameters can then be adjusted to prevent the saturation demagnetizingfield from being as small as the maximum value in that range.

Operating the circuit of FIG. 6A with such a shield over each of theresistors R₂ would, from the equation therefor given above assuming noeffective bias field present, leave them exhibiting the resistance valueR_(o) substantially independent of the applied magnetic field, whetherapplied in the positive or negative direction, because of the shieldthereover. The resistance values of resistors R₁, also assuming noeffective bias field present, for positive direction applied fields,would, from the equation above be

    R.sub.1 =R.sub.o -KH.sub.a.

A review of the resistance characteristic in FIG. 8A, ignoring the biasfield shown there, shows the negative field portion of thecharacteristic would be represented by the same equation since the slopeof that portion is positive. Substituting these results into theequation given above for the signal voltage v_(s) in the circuit of FIG.6A yields ##EQU22## which, for R_(o) much greater than KH_(a), issubstantially linear with |H_(a) |. Thus, such a sensor using the bridgecircuit of FIG. 6A with two of the structures, or resistors, shielded isan absolute value applied magnetic field sensor.

As the concentration factor found above did not depend strongly on theconcentrator thickness, and as the shielding requirements do dependsignificantly on the shield thickness, a very compact, high performancemagnetic sensor can be fabricated for sensing magnetic fields,especially fields smaller in magnitude than the fields which wouldsaturate the giant magnetoresistive effect resistance characteristic forthe structures described above. If two separated magnetic materialmasses are provided in shapes suitable for use as flux concentrators fortwo of the giant magnetoresistive effect structures, or resistors, inthe circuit of FIG. 6A, one of those same masses may concurrently bealso be provided in a shape suitable for an additional use as a magneticshield for the other two structures or resistors in that circuit. Such astructural arrangement is shown in FIGS. 9A and 9B.

The device shown in plan view in FIG. 9A is formed as a monolithicintegrated circuit on semiconductor chip substrate 10, 11 so operatingcircuitry can be conveniently provided therewith, but couldalternatively be formed as a hybrid integrated circuit on a ceramicsubstrate. A concentrator-shield, 30A, and a concentrator, 30B, formedof permalloy (typically comprising around 80% nickel and 20% iron toprovide approximately zero magnetorestriction for these masses on thesupport layer therefor over the resistors provided on substrate 10, 11)are shown in that figure over a part of the integrated circuitry in thatdevice which is an implementation of the bridge portion of the FIG. 6Acircuit schematic diagram. The optional protective layer that can beprovided over these concentrators has been omitted in this view forclarity so they are shown in solid line form, but the direct sensorcircuitry below them is indicated in dashed line form (no integratedcircuits in substrate 10,11 are shown) except at the opening overresistors R₂ and in solid line form elsewhere.

Each of the two resistors R₁ is positioned in the gap between, andsomewhat below, concentrator-shield 30A and concentrator 30B, and formedin serpentine fashion with each of the many successive major lengthportions thereof, typically just under 90 μm long, being parallel toeach other and to the long axis of concentrator-shield 30A andconcentrator 30B to thereby be aligned with the concentrated fieldcomponent resulting therefrom along this axis. Only very shortconnections perpendicular to this axis are used to interconnect the endsof such a resistor length to the corresponding ends of one or the otherof adjacent such lengths such that the dimension of the resistorconsidered as a whole in the direction perpendicular to the resistorlengths is somewhat less than that of such a resistor length.

An alternative arrangement for resistors R₁ is shown in FIG. 10 wherethese two resistors are shown "interleaved" with one another. Such anarrangement can result in a better match of characteristics betweenthese two resistors if instead used but at the cost of providing acrossover for the interconnecting circuitry.

Resistors R₂, on the other hand, are positioned underconcentrator-shield 30A below more or less the center thereof butotherwise formed in the same manner as are resistors R₁. There,resistors R₂ are substantially shielded from applied magnetic fieldcomponents occurring along the long axis of concentrator-shield 30A andconcentrator 30B. Resistors R₂ could also alternatively be interleavedwith one another in the manner, for instance, shown in FIG. 10.

Typical dimensions for concentrator-shield 30A and concentrator 30B inFIG. 9A are a length of 405 μm, a width of 250 μm, and each having athickness of 12.5 μm. Concentrator-shield 30A, as a result of thefabrication process result. is located approximately 2 μm aboveresistors R₂. The gap separating concentrator-shield 30A andconcentrator 30B is typically just a bit greater than 90 μm. On the gapend of each of concentrator-shield 30A and concentrator 30B a 1:2 taperhas been used to slightly narrow the gap ends, each edge of each gap endbeing 25 μm inward from the corresponding one of the main sides with thetapering beginning 50 μm back from the gap ends. Typical dimensions ofthe integrated circuit chip serving as the sensor substrate would be 77mils by 18 mils.

The thickness and position separation dimensions just given can bebetter seen and understood in FIG. 9B which is a fragmentary crosssection view of a portion of FIG. 9A taken at a position to show part ofa resistor R₂. This view also helps to understand the fabricationprocess in showing some of the structures resulting from practicing thesteps of that process for which the main steps will now be described.

Silicon wafer 10 has about 10,000 Å of silicon nitride 11 provided on itby sputter deposition to form substrate 10,11. A giant magnetoresistivemultilayer, having a sheet resistivity of about 12 Ω/□, or higher, witha giant magnetoresistive effect exceeding 10% and a saturation field ofabout 200 Oe, is sputter deposited, as described above, onto nitride 11in a sputter deposition machine as the basis for the circuit resistors.This multilayer (which will be about 400 Å thick) is passivated with 100Å of sputter deposited tantalum which also serves as an interconnectioninterface. The wafer is then covered with 100 Å of chromium silicide bysputter deposition which is to be used as an etch stop, and then by 1500Å of silicon nitride again by sputter deposition. A first maskdelineates the multilayer structures being chosen to serve as resistorsR₁ and R₂ on the basis of using a reactive ion etching step to etchthrough this last deposited silicon nitride to the chromium silicide,serving as indicated as an etch stop, in those locations in whichstructures for such resistors are not to be present. Then an ion mill isused to cut through the chromium silicide, the tantalum and themultilayer, with about 300 Å overreach to insure complete etching, tothus form the structures for these resistors with the tantalum remanentthereon as interconnection interface 13'. There will also be a chromiumsilicide layer thereon, and some silicon nitride residue over thischromium silicide layer remaining on the protected resistor structures.

The wafer is then covered with a further 1500 Å of silicon nitridethrough sputter deposition, and a second mask with another reactive ionetching step opens contact vias to the chromium silicide and tantalumlayers of the multilayer resistor structures for electricallyinterconnecting these structures (and to interconnection provisions madefor interconnections to the circuits in the integrated circuittherebelow in semiconductor wafer 10). There will be left a chromiumsilicide layer, 31, on the structures with some remanent portion thereofin the vias, on which will reside a silicon nitride remanent, 32, aboutthe vias including the previous silicon nitride residue, 32'. A 5000 Åthick aluminum film alloyed with 2% copper is then sputter-deposited onthe resulting construct, and this film is etched using a third masklayer to leave it in electrical contact with the resistor structuresthrough the vias and to leave it providing the desired electricalinterconnection arrangement for the device, as shown in FIG. 9A. Thisetching again accomplished by a wet chemical etching step to result inthis interconnection arrangement, 33.

A further layer of 10,000 Å of silicon nitride, 34, is then sputteredonto this interconnection result. This last provided silicon nitridelayer is to serve as the base for concentrator-shield 30A and shield 30Bto be provided by a thick (0.5 mil) permalloy material plated thereon.Such plating begins with a 1000 Å plating seed film of permalloy whichis sputtered onto this silicon nitride base. This initially platedresult is then coated with a thick layer of photoresist which is exposedand developed to define the regions where the main portions ofconcentrator-shield 30A and concentrator 30B are to be plated. This isfollowed by chemically plating a 4 μm thick layer of gold on the exposedportions of the permalloy seed plating. The gold improves adhesion ofthe main portions to be provided. Such main portions are then chemicallyplated onto the exposed gold to a thickness of 12.5 μm to provideconcentrator-shield 30A in FIG. 9B.

The developed photoresist is then removed, and the resulting constructis then dipped into a bath formed of a mixture of phosphoric, asceticand nitric acids to remove the 1000 Å permalloy seed layer from allareas not under either concentrator-shield 30A or concentrator 30B. Afinal mask then defines bonding pads to be provided, and vias thereforto interconnection arrangement 33 are provided using this mask and areactive ion etching step. This last etching step can be preceded by afurther sputter deposition of silicon nitride for protective purposes ifdesired providing the layer shown in dashed lines and designated 35 inFIG. 9B. The wafer after such provision of bonding pad vias is thenready for wafer probe, dicing, and packaging.

The devices resulting from such a fabrication process have providedshielding to the resistors R₂ sufficient to reduce the magnetic fieldoccurring therein by around 95% over the ambient field. In addition, theresistors R₁ have had the field occurring therein concentrated by afactor of 5 so that they are in a field of a value some 5 times that ofthe ambient field. Thus, the device of FIGS. 9A and 9B is an effectivesensor of magnetic field components aligned along the axis passingthrough concentrator-shield 30A and concentrator 30B and the gaptherebetween.

The structural arrangement of FIG. 9A must have concentrator-shield 30therein, provided over resistors R₂, with sufficient dimensions asindicated above to result in it having a saturation demagnetizing fieldthat is greater than the expected maximum externally magnetic appliedfield if the permeable material mass forming same is to be sufficient toshield those resistors from being significantly affected by suchexternally applied fields. Of course, simultaneously with such effectiveshielding that mass must aid in providing the desired concentrating ofexternally applied magnetic fields. Permeable material masses ofrelatively large length in the direction of the externally appliedfields provide substantial concentration and shielding but saturate inrelatively small externally applied fields, as indicated above. On theother hand as also indicated, such masses which are relatively short inthat direction will not saturate until substantially higher appliedfield values are reached but will not provide satisfactory shielding orconcentration. In many instances a satisfactory design can be achievedby increasing the thickness dimension of the permeable material masswhile choosing an adequate length dimension as set out above.

However, in a significant number of situations, the ability to form suchpermeable material masses appropriately sized on an integrated circuitchip to provide the assurance that they will not saturate in externallyapplied magnetic fields of the expected maximum value while providingsufficient shielding and concentration may be lacking. This situationwould typically come about because of the fabrication processconstraints or economic constraints, or both, that must also besatisfied. In such situations, an alternative permeable material massarrangement can be used as is shown in FIG. 11 which will result in bothsufficient shielding and concentration while avoiding saturation as lowapplied fields, and can accomplish this result using masses ofacceptable dimensions on an integrated circuit chip.

There in FIG. 11, concentrator-shield 30A of FIG. 9A has had thepermeable material mass forming it split into two portions including aconcentrator portion, 30A', and a shield portion, 30A". Permeablematerial masses 30A' and 30A" are separated by a gap, 40, which has ashape somewhat like that of a "U", i.e. like that of a portion of arectangle with one of the shortest sides thereof missing. As a result,concentrator permeable material mass 30A'extends from the gap separatingconcentrator 30A' from concentrator 30B, at which gap resistors R₁ arelocated, back toward shield permeable material mass 30A" provided overresistors R₂ to partially surround that shield as can be seen in FIG.11. That is, although the bulk of concentrator 30A' is provided betweenshield 30A" and the gap between concentrators at which resistors R₁ areprovided, extension portions, 41 and 42, of concentrator 30A' extendfurther back past that edge of shield 30A" closest to resistors R₁ to bepositioned along either side of shield 30A" to contain that shieldbetween them.

This configuration results in the permeable material mass for shield30A" having a much shorter dimension of extent along an axis passingthrough concentrators 30A' and 30B, including through the gaptherebetween, and extending further across gap 40 through shield 30A".As a result, there is a much larger demagnetizing field present inshield 30A" along that axis in response to an externally applied fieldtherealong resulting in a smaller net field therein compared to the netfield resulting in shield-concentrator 30A of FIG. 9A from the samevalue of externally applied field. Thus, a considerably larger externalfield will be required to saturate shield 30A" in the structure of FIG.11 than would have been required to saturate shield-concentrator 30A inthe structure of FIG. 9A.

On the other hand, there will be some reduction in the magnetic field towhich resistors R₁ in the gap between concentrators 30A' and 30B issubject as the effective length of the concentrating permeable materialmass is reduced. This is offset in part by the presence of extensions 41and 42 of concentrator 30A' on either side of shield 30A" which tend toserve as flux guides around both shield 30A" and resistors R₂ positionedtherebelow. Thus, the reduction in concentration will not be too great,and will thus result in acceptable sensitivity for the sensor whileallowing it to respond to greater externally applied fields without lossof the effective shielding of resistors R₂ from that field.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A magnetic field sensor comprising:a substratehaving a major surface portion; a pair of magnetoresistive thin-filmlayered structures provided on said substrate electrically connected toone another and with each also being electrically connected to aninterconnection means suited for electrical connection to a source ofelectrical energy; and a group comprising three permeable materialmasses provided on said substrate with one of said two layeredstructures being positioned adjacent a first gap between a first andsecond of said permeable material masses in said group thereof, and withthat layered structure remaining positioned (a) adjacent a side of athird of said permeable material masses in said group thereof which sidefaces said substrate as supported thereon, but (b) spaced apart fromsaid first gap and from a second gap between said second and thirdpermeable masses in said group thereof in a direction at least in partparalleling an axis substantially parallel to said substrate passingthrough both of said second and third permeable material masses and saidsecond gap.
 2. The apparatus of claim 1 wherein said second gap followsa plurality of directions along its course as determined by boundariesof said second and third permeable material masses.
 3. The apparatus ofclaim 2 wherein said second permeable material mass at least partiallysurrounds said third permeable material mass.
 4. The apparatus of claim1 wherein said layered structures are first and second structures eachcomprising a succession of layers including a magnetoresistive,anisotropic, first ferromagnetic thin-film provided on said substratemajor surface portion; an intermediate layer provided on said firstferromagnetic thin-film with a thickness of less than 50 Å, saidintermediate layer being formed of a substantially non-magnetic,conductive alloy having two substantially mutually immiscible componentstherein and a third component therein at least partially immiscible witheach of those first two components; and a magnetoresistive, anisotropic,second ferromagnetic thin-film provided on said intermediate layer. 5.The apparatus of claim 4 wherein said layered structures areelectrically connected in a bridge circuit.
 6. The apparatus of claim 1wherein said layered structures are first and second structures eachcomprising a succession of layers including a magnetoresistive,anisotropic, first ferromagnetic thin-film provided on said substratemajor surface portion; an intermediate layer provided on said firstferromagnetic thin-film with a thickness of less than 50 Å, saidintermediate layer being formed of a substantially non-magnetic,conductive alloy having copper, silver and gold components therein; anda magnetoresistive, anisotropic, second ferromagnetic thin-film providedon said intermediate layer.
 7. A magnetic field sensor comprising:asubstrate having a major surface portion; a pair of magnetoresistivethin-film layered structures provided on said substrate electricallyconnected to one another and with each also being electrically connectedto an interconnection means suited for electrical connection to a sourceof electrical energy, said pair of magnetoresistive thin-film layeredstructures comprising magnetoresistive, anisotropic, first and secondferromagnetic thin-films separated by the intermediate layer having athickness of less than 30 Å and formed of a substantially non-magneticconductive alloy having two substantially immiscible components therein;and a pair of permeable material masses provided on said substrate withone of said two layered structures being positioned in a gaptherebetween, and with that one remaining positioned near a side of oneof said permeable material masses between surface locations thereonintersected by an axis substantially parallel to said substrate passingthrough both said permeable masses and said gap.
 8. The apparatus ofclaim 7 wherein said two substantially immiscible components are copperand silver.
 9. The apparatus of claim 7 wherein said two substantiallyimmiscible components are copper and gold.
 10. The apparatus of claim 7wherein there is a third permeable material mass positioned in said gapto thereby form a pair of subgaps so that said layered structurepositioned in said gap is also positioned in a first of said pair ofsubgaps located between said third permeable material mass and one ofsaid pair of permeable material masses with a second of said pair ofsubgaps positioned between said third permeable material mass and thatremaining one of said pair of permeable material masses.
 11. Theapparatus of claim 10 wherein said two substantially immisciblecomponents are copper and silver.
 12. The apparatus of claim 10 whereinsaid two substantially immiscible components are copper and gold.
 13. Amagnetic field sensor comprising:a substrate having a major surfaceportion; a pair of magnetoresistive thin-film layered structuresprovided on said substrate electrically connected to one another andwith each also being electrically connected to an interconnection meanssuited for electrical connection to a source of electrical energy, saidpair of magnetoresistive thin-film layered structures comprisingmagnetoresistive, anisotropic, first and second ferromagnetic thin-filmsprovided on said substrate major surface portion separated by anintermediate layer with a thickness of less than 30 Å and formed of asubstantially non-magnetic, conductive alloy having copper and silvercomponents therein; and a pair of permeable material masses provided onsaid substrate with one of said two layered structures being positionedin a gap therebetween, and with that one remaining positioned near aside of one of said permeable material masses between surface locationsthereon intersected by an axis substantially parallel to said substratepassing through both said permeable material masses and said gap. 14.The apparatus of claim 13 wherein there is a third permeable materialmass positioned in said gap to thereby form a pair of subgaps so thatsaid layered structure positioned in said gap is also positioned in afirst of said pair of subgaps located between said third permeablematerial mass and one of said pair of permeable material masses with asecond of said pair of subgaps positioned between said third permeablematerial mass and that remaining one of said pair of permeable materialmasses.
 15. A magnetic field sensor comprising:a substrate having amajor surface portion; a pair of magnetoresistive thin-film layeredstructures provided on said substrate electrically connected to oneanother and with each also being electrically connected to aninterconnection means suited for electrical connection to a source ofelectrical energy, said pair of magnetoresistive thin-film layeredstructures comprising magnetoresistive, anisotropic, first and secondferromagnetic thin-films provided on said substrate major surfaceportion separated by an intermediate layer with a thickness of less than30 Å and formed of a substantially non-magnetic, conductive alloy havingcopper and gold components therein; and a pair of permeable materialmasses provided on said substrate with one of said two layeredstructures being positioned in a gap therebetween, and with that oneremaining positioned near a side of one of said permeable materialmasses between surface locations thereon intersected by an axissubstantially parallel to said substrate passing through both saidpermeable material masses and said gap.
 16. The apparatus of claim 15wherein there is a third permeable material mass positioned in said gapto thereby form a pair of subgaps so that said layered structurepositioned in said gap is also positioned in a first of said pair ofsubgaps located between said third permeable material mass and one ofsaid pair of permeable material masses with a second of said pair ofsubgaps positioned between said third permeable material mass and thatremaining one of said pair of permeable material masses.